1. Field of the Invention
The present invention relates to a drive circuit for a static induction transistor and, more particularly, to a semiconductor device drive circuit for controlling the driving of a semiconductor device comprised of a main static induction transistor and a sense static induction transistor by a sense current flowing through the sense static induction transistor.
2. Description of the Related Art
Static induction transistors have been developed and used as high-power switching devices, particularly as low-voltage high-current switching devices. Although such static induction transistors (SITs) can handle low voltage and high current, they may suffer breakdown if, for example, a failure (a short) occurs in a load. Thus, an SIT is designed such that a main SIT and a sense SIT are combined into a single semiconductor device. Their drains are connected to a common drain terminal, and their gates and sources are connected to their respective terminals. Thus, the SIT has a five-terminal structure. In order to avoid the breakdown of SITs, a method is frequently used in which the entire current is caused to flow through the semiconductor device by use of a current flowing through the sense SIT and the main SIT and the sense SIT are turned off when a current higher than a specific value flows through the sense SIT.
FIG. 1 illustrates the configuration of a prior art circuit. Resistors R.sub.G and R.sub.GS each have one of their ends connected to a gate terminal of a semiconductor circuit (a semiconductor device) comprising a main SIT and a sense SIT. They have their other ends connected to the positive terminal (Vcc) of a power supply B via a switch SW. Connected to an input signal source S is a control circuit 10 which, when an input signal thereto reaches a high-level, turns switch SW on to supply currents I.sub.G and I.sub.GS to the gates of the semiconductor circuit through resistors R.sub.G and R.sub.GS, thereby turning on the main SIT and sense SIT. As a result, a current I.sub.D flows through a load R.sub.L. At this time, namely, when the main SIT and the sense SIT are turned on, the ratio of the current flowing through the main SIT to the current flowing through the sense SIT is k:1. The main SIT has its source connected to the negative terminal of power supply B and the sense SIT has its source connected to the negative terminal of power supply B via a resistor R.sub.S. Since the ratio of the main SIT current to the sense SIT current is k:1, the voltage (Vs) developed across resistor R.sub.S connected to the source of the sense SIT will give a measure of the entire current flowing through the semiconductor device. In case where, for example, load R.sub.L is defective and hence shorted, current increases and voltage Vs across resistor R.sub.S also increases. Voltage Vs is applied to an overcurrent protection circuit 11 which compares the magnitude of Vs with a specific value. When the voltage Vs is higher than the specific value, namely, when a current sufficient to cause the breakdown of the semiconductor device flows through it, overcurrent protection circuit 11 prompts control circuit 10 to turn switch SW off.
The above operation enables the prior art circuit to avoid the breakdown of the semiconductor device, which serves as a switching device, that is, due to a defect in the load.
When the semiconductor device is used for a switch, however, the sense SIT and the main SIT each have to be driven to saturation. The reason for this is as follows. With the sense SIT nonsaturated (for example, when supply voltage is low), even if the load is shorted and hence an excessive current flows through the main SIT, it cannot be sensed and thus the main SIT suffers breakdown. With the sense SIT saturated and the main SIT nonsaturated, on the other hand, no excessive current will flow through the main SIT even when the load is shorted. During normal use of the semiconductor device, however, the main SIT cannot be completely turned on and the sense SIT may suffer breakdown.
In the conventional breakdown preventing system, a current is sensed by use of the sense SITs of the semiconductor device, thus turning it off, when a current exceeding the maximum rating flows. The current I.sub.GS flowing into the gate of the sense SIT will vary with the supply voltage Vcc of power supply B because the gates of the device are biased through resistors R.sub.S and R.sub.GS. For this reason, the current flowing into the sense SIT will be low even if a short is caused by a defect in load resistor R.sub.L. As a result, voltage Vs across resistor R.sub.S may be lower than the specific voltage. Thus, a problem with the prior art is that, when the supply voltage is low, an excessive current cannot be detected even if the load is defective, thereby causing the breakdown of the semiconductor device. In other words, with the prior art circuit using the semiconductor device, the sense SIT is not completely turned on unless it is sufficiently saturated and the sense voltage Vs is too low to detect an excessive current even if load R.sub.L is shorted. Furthermore, the lowered supply voltage will also make I.sub.GS low, thus activating only the sense SIT and causing its breakdown when load R.sub.L is shorted.